The invention relates to a pilot-operated pressure-limiting valve with a feeding function wherein a main control spool, when in its pressure limiting function, is acted upon by operating medium flowing in its opening direction and when in its feeding function, is acted upon by pressure medium flowing in a direction transverse to the opening direction. Such pressure-limiting valves are installed in the known manner within closed hydraulic circuits, wherein the feed-in of operating medium or hydraulic liquid, is implemented at the low-pressure side by a feed pump and through the pressure-limiting valve. Feeding will ensue herein against the direction of flow prevailing during pressure-limiting operation.
In pressure-limiting operation, the operating medium will impinge upon the face of the main control spool which is constructed as a support in the opening direction of its movement, while during feeding operation, the operating medium will laterally impinge upon the main control spool. In order to allow opening of the main control spool by the operating medium being fed in, it must be provided with an annular area to be impinged upon by the operating medium flowing in the opening direction of the main control spool. This additional annular area will increase the effective area of the main control spool available for acting in the closing direction. In consequence thereof, the main control spool will, during pressure-limiting operation, not open simultaneously with the pilot valve. For the main control spool to open, the pressure before the main control spool must be higher, by a differential corresponding to the annular area, than the pressure acting in the closing direction and holding the pilot valve in the open position. This preliminary opening of the pilot valve relative to the main control spool will cause a loss of efficiency in the hydraulic drive. This loss in efficiency will be particularly large when the hydraulic drive operates for a longer period within the pressure range that causes preliminary opening of the pilot valve.
In order to avoid such uneconomical operation of a pressure-limiting valve with a feeding function, it has already been proposed in DT published patent application No. 26 31 922, to have the main control spool laterally impinged by high pressure, and to have the low pressure act against the face, i.e. in the direction of opening of the main control spool. Therein, the pilot cone is provided with a piston-shaped extension in order to form an area upon which pressure can act to effect opening of the pilot valve. This extension is located within an axial bore of the main control spool, the axial bore being connected to a lateral bore, the extension thus forming, conjointly with the axial bore, the requisite restrictor point before the control space of the main control spool.
In such a construction of the pressure-limiting valve with feeding it, is of the essential disadvantage that no precisely definable shape can be achieved for the restriction between the piston-shaped extension of the pilot cone and the bore of the main control spool, so that fluctuations will thus be caused in the intermediate range which, finally, will cause system-pressure fluctuations. Furthermore, such an annular-gap restrictor will lead to the preset pressure becoming temperature-sensitive. Finally, unavoidable friction of the piston-shaped extension within the bore of the main control spool will impair a constant pressure level. A further deficiency results from lateral impingement of the operating medium upon the main control spool during pressure-limiting operation, i.e. on acount of a negative attenuating length at the main control spool which, during certain operating conditions, will cause instability, i.e. fluttering, of the main control spool. A further disadvantage may also be seen in the fact that it is not possible to conduct the relieved high-pressure oil within the valve to the low-pressure side. It is, on the contrary, necessary to provide additional return passages within the base plate for the pressure-limiting valve, valve usually being constructed as insertable cartridge.